Lithium ion containing glass (lithium glass) is becoming more desirable for use in portable electronic devices. As the use of lithium glass increases in these smaller and higher tolerance electronic devices, fabrication of thinner and stronger lithium glass is required. One such lithium glass is sodium strengthened lithium glass, a lithium glass that includes a surface strengthened with sodium ions.
Thin layers of sodium strengthened lithium glass can be formed by diffusing sodium ions into the surface of the lithium glass, where the smaller lithium ions are exchanged for much larger sodium ions. Exchange of smaller for larger radius ions results in the formation of a diffusion layer or compression layer, often 50 to 300 μm in thickness. As such, sodium strengthened lithium glass typically includes an internal portion and a coated compression layer.
Defects in the compression layer can have a significant impact on the utility of the lithium glass, as well as on the utility of the devices that incorporate the glass. Defects can either be inherent to a particular piece of glass or be introduced to the compression layer during post-processing events, like touch polishing. Removal or sacrifice of material from the surface of the glass can result in inconsistent and weakened compression layers.
Once a portable electronic device has been manufactured, a defective glass article can result in the loss of the entire product. Identifying defects in sodium strengthened lithium glass represents a significant concern in the art, as there are no practical non-destructive methods for identifying defects in a compression layer. As such, there are no non-destructive methods for identifying defective sodium strengthened lithium glass.